Ink writing head with piezoelectrically excitable membrane

ABSTRACT

An ink writing head comprising ink ejection channels and piezoelectric transducer elements allocated to the ink ejection channels, these transducer elements being supplied with writing fluid via supply lines, contains transducer elements comprising a first piezoelectrically excitable layer and a supporting layer firmly joined to the excitable layer. The piezoelectrically excitable layer comprises deflectable regions that are respectively subdivided into a peripheral and into a central region. For generating the needed excursion of the membrane, the regions of the membrane are driven such that the peripheral region is shortened, preferably by transversal contraction, and the central region is lengthened.

The invention is directed to an in writing head and to a method for themanufacture of an ink writing head according to the preamble of patentclaims 1 and 13.

Piezoelectrically operated drive elements in ink printers are generallyknown. Thus, German Published Application No. 21 64 614 discloses anarrangement in printing units for writing on paper with colored fluidwherein a fluid situated in an ink chamber is ejected from a printer jetvia a piezoelectrically operated drive element. The volume change in thechamber is effected by an electrically driven piezoceramic that isseated on a metal plate and that arcs into the chamber. The employedpiezo drive element is composed of a continuously polarized piezoceramiclayer that is arranged on a metal plate, whereby the metal plate servesas cooperating electrode. When a suitable voltage pulse is applied, thepiezoceramic constricts. Since the ceramic is secured to a metal plate,a bending moment acts on this plate. This results therein that themiddle part of the plate arcs into the fluid chamber.

The length changes that can be directly piezoelectrically produced aredisappearingly small. They are also limited by the electrical fieldstrengths that dare be applied to the ceramic without this leading topunch-throughs or arc-overs. Further, the applied field strengths darenot lead to a re-polarization; they must also be switchable viaappropriate drive circuits.

It is therefore standard to not exceed a voltage of about 200 V. Thefield strength should thereby be lower than 1 V/μm in the directionopposite the polarization. The distances between electrodes at air,moreover, should not be smaller than 1 μm/V. The direct length changethat can be achieved in this way is thus about 0.1% or about 0.2 μmgiven a layer thickness of 200 μm, assuming that the ceramic is activethrough and through and is not partly inactive due, for instance, to afiring skin. In ink printing the drive elements, whether they are smallpiezo tubes or piezo laminae, are needed for a whole series offunctions. They should accelerate controllably small ink quantities,eject them as droplets and replenish ink from a reservoir. If possible,however, they should also close the ejection openings in order toprevent the runout and the drying of the ink. Finally, the ink channelsand the discharge openings should be capable of being cleaned andaerated with such elements.

Only a part of these functions are fully met in the known drive elementscomprising acoustic drop formation. Sound waves in the ink channel canin fact form rapidly flying drops, but static pressure for eliminatingimpediments in the channel cannot be generated. Air inclusions in theink channel limit the propagation of the pressure waves in the channeland channels that have emptied can only be refilled with an outsideintervention. Given acoustic drop formation, the closure of thedischarge openings can likewise only ensue mechanically from theoutside.

It is therefore an object of the invention to fashion an ink head of thespecies initially cited such that, first, it can be simply manufacturedin a galvanoplastic method and that, second, it exhibits a high degreeof efficiency.

In an apparatus of the species initially cited, this object is achievedin accord with the characterizing part of the first patent claim.

Advantageous embodiments of the invention are characterized in thesubclaims.

An especially large stroke derives in that the membrane comprises apiezoelectrically excitable peripheral region and a piezoelectricallyexcitable central region that are driven such for producing a membraneexcursion that the membrane is shortened in its peripheral region bytransversal contraction and is lengthened in its central region. Thisstroke is the result of exploiting two actions, namely the exploitationof the transversal contraction in the ceramic itself and the curvatureof layers adjacent to the composite that dilate differently. Due to thetransversal contraction, the stroke of the membrane can be increased byreducing the layer thicknesses and enlarging the length dimensions.

An especially advantageous dynamic action derives when the membraneregions are arranged concentrically relative to one another so that theyarc outward button-like when excited. This button-like convexityrepresents the smallest and most compact geometrical shape that proceedsfrom a planar layer and expands and closes a cavity. It is dynamicallybalanced around a surface normal and departs the plane in a torus-shapedchamfer that merges into a lens-shaped segment of a sphere. The requiredcurvature condition changes at the transition line. Accordingly, theelectrodes are arranged such or, respectively, the correspondingmembrane regions are polarized such and driven via the electrodes thatthe peripheral region (annulus) shortens but the central regionlengthens. The edge of the membrane does not change its slope uponexcursion, for which reason it can be firmly clamped. The elastic lineessentially corresponds to an excursion under inside pressure. In afurther, advantageous embodiment of the invention a plurality ofmembranes individually activatable independently of one another arearranged on a common substrate surface, whereby the drive lines for theindividual membrane regions lead across unpolarized regions of thesubstrate surface so that no undesired piezoelectric effects appear viathese drive lines during driving.

In order to further increase the stroke, the supporting layer can bereplaced by a further piezoelectrically excitable layer that isrespectively polarized is a direction opposite the firstpiezoelectrically excitable layer. Nearly a doubling of the stroke thusderives.

An especially simple ink writing head that operates with operationalreliability can be fashioned in that every ink channel has threemembranes that are connected to one another via a pump channel for thewriting fluid allocated to it, these membranes forming a static pumpcomprising two controllable rotary pistons and a variable cavity. Thefirst membrane region communicates, first, with the ink supply systemvia a supply channel and, second with the variable cavity and serves asadmission valve. The second membrane region is allocated to the variablecavity and a third membrane region is arranged between the cavity andthe ink channel as outlet valve. In an advantageous embodiment of theinvention, the pump channel that connects the membranes comprisingparting webs in the region of the membranes fashioned as valves, theseparting webs interacting with the membrane surfaces such that, after theexcusrion of the membrane surfaces, the pump channel opens via theparting webs and, in the non-deflected condition of the membranesurfaces, the pump channel is interrupted in the region of the partingwebs via the membrane surfaces.

The parting webs can thereby be fashioned as through webs or can also befashioned as collar-like elevations having outlet openings or,respectively, admission openings lying therebetween.

Since ink having low viscosity can be used in the ink writing head ofthe invention, the ink can be filtered considerably better, thepenetration of dirt into the ink channels being therewith avoided. It isalso additionally possible to electrically expand the transmissioncrossection for cleaning purposes and to reverse the pump direction.Moreover, the parting web can also be directly cleaned with ultrasoundand contaminations can be ground up at the parting web.

Since the transducer elements keep the ink channels closed as long asthe transducer elements are not driven, a mechanical closure of thenozzles is not necessary between the writing head and the paper and thedrive of such a closure can be eliminated. It is thus possible togreater reduce the distance from the paper, wherewith the print image isless negatively affected by the scatter of the flight speed and of theflight direction of the drops. Since the pressure can be staticallyimpressed on the nozzle, the flight speed can be elevated. A crosstalkbetween the nozzles is eliminated since there is no flow connectionduring spraying.

The ejection frequency is not limited by reflections in the channel andis not limited by the crosstalk of neighboring nozzles but is limitedonly by the intrinsic values of the individual transducer elements. Anindividual balancing of the transducer elements can be omitted since thecoupling of the transducer to the ink ensues far more directly anduniformly.

Since the ink supply system of the invention is independent of staticunderpressure, it becomes significantly more insensitive, thesensitivity of the ink writing head to acceleration also disappearingtherewith.

Air bubbles can be eliminated from the ink channel by static pumping.Empty channels can be filled under electrical control.

The ink reservoir can be stationarily accomodated in the printer withoutdifficulty. Pressure waves from the moving supply hose do not act on thedrop formation.

The monitoring of the ink supply is no longer bound to the narrow limitsof a static pressure in the reservoir.

The overall writing head can be manufactured in planar technology in anespecially simple way. The critical part, namely the piezoceramic, canbe tested before the actual assembly.

Embodiments of the invention are shown in the drawings and shall be setforth in greater detail below by way of example. Shown are:

FIG. 1 a schematic comparison of the deformation of a membrane plateunder inside pressure and a membrane plate having impressed convexity;

FlG. 2 a membrane of the invention in its deflected condition;

FIG. 3 a membrane of the invention in its unexcited condition;

FIG. 4 a static pump composed of three membranes connected to oneanother, shown in a plan view;

FIG. 5 a static pump of FIG. 4 shown in crossection;

FIGS. 6 through 10 schematic illustrations of the layer format of theink writing head of the invention;

FIG. 11 a schematic, sectional view of a transducer element comprisingcollar-shaped parting webs;

FIG. 12 a schematic illustration of the ink writing head of theinvention; and

FIG. 13 a schematic illustration of the oblique positioning of the inkwriting head in a line printer means.

FIG. 14 is a schematic illustration of the oblique position of an inkprinting head in a line printing means.

A planar transducer of piezoceramic as shown in FIGS. 2 and 3 iscomposed of a piezoelectrically excitable layer 1 of piezoceramic thatis continuously polarized in one direction and is further composed of asupporting layer 2 of, for example, nickel that is firmly joined to thisexcitable layer. The electrically drivable membrane formed in this wayis driven via corresponding electrodes 3,4, whereby the supporting layer2 serves as a through electrode to ground and the actual driveelectrodes are composed of a peripheral drive electrode 3 and of acentral drive electrode 4. These drive electrodes 3 and 4 definemembrane regions is the shape of circular areas and, respectively,annular areas that are arranged concentrically relative to one another.When a membrane constructed in this way is then driven such thatelectrical fields that differ in direction are formed between theelectrodes 3 and 4 of the polarized piezoceramic 1 and the commoncooperating electrode 2, then the membrane arcs outward in the directionshown in FIG. 2 when the annular electrode 3 causes a contraction of thepiezoceramic layer 1 in the region of the annular electrode 3 and adilation of the piezoceramic layer 1 arises in the region of theelectrode 4.

This shall be set forth in greater detail below with reference to FIG.1.

The smallest and most compact shape that proceeds from a planar surface,requires only weak curvatures and expands and closes a cavity is abutton or a dome-like convexity. Such a shape is dynamically balancedaround a surface normal and leaves the plane in a torus-shaped chamferthat merges into a lens-shaped segment of a sphere.

Such an ideal shape can then be generated in that a planar, elasticmembrane is subjected to a uniform inside pressure. The shape shown atthe left-hand side of FIG. 1a thereby derives having the slope shown inFIG. 1b and a curvature according to FIG. 1c, whereby the abscissa isallocated to the radius of the membrane area.

In order to achieve this ideal button shape, the drive electrodes 3 and4 are fashioned such in combination with the piezoelectrically excitablelayer 1 and the supporting layer 2 that serves as electrode to groundthat this ideal shape approximately derives given excursion.

To this end, the circular outside electrode 3 is arranged in the outercurvature region of the membrane and is charged with such an electricalfield that the piezoelectric layer contracts in this curvature region.The inside electrode 4 arranged concentrically relative thereto ischarged with such a field that the central region of the piezoceramiclayer 1 dilates. Two effects are thus simultaneously exploited, namelythe transversal contraction of the ceramic itself and the curvature oflayers adjacent to the composite that expand differently. The radius ofcurvature up to which planar layers can be arced in this way lies atabout 0.1 m through 0.4 m dependent on how thin the layers can be made.The ratio of the electrode areas to one another is then dimensioned suchthat the desired approximation of the course in FIG. 1a derives. Thisyields a slope according to FIG. 1b having the appertaining curvature ofFIG. 1c (right-hand side of FIG. 1).

As shown in FIGS. 2 through 5, a static pump composed of twocontrollable rotary pistons SE and SA and of a variable cavity H can befashioned with such a planar transducer of piezoceramic. To this end,three membrane regions SE,H,SA are fashioned in a ceramic substrate. Apump channel P is fashioned in a carrier layer T that carries thesubstrate 1 together with its appertaining supporting layer 2. This pumpchannel P is in communication with a fluid supply V (FIG. 4). Across-rib Q is applied in the pump channel in the region of theadmission valve SE, the membrane of piezoceramic 1 and supporting layer2 lying against this cross-rib Q in its unexcited condition and thusclosing the channel. In the excited condition corresponding to FIG. 2,the membrane lifts off button-shaped and thus opens the channel P.

The same structure as at the admission valve SE having the cross-rib Qderives at the outlet valve SA having the cross-rib Q there. Themembrane region H that is constructed in conformity with the membranesof the admission valves [sic] SE and SA and serves as the actual pump His situated between the admission valve SE and the outlet valve SA. Apump constructed in this fashion, as shown in FIGS. 4 and 5, can then bedriven in an advantageous way, for example via a three-phase current,namely in that the admission valve SE is first opened with a first phasein a pumping step, in that fluid is taken in from the supply V due toexcursion of the membrane H, and in that, after the admission valve SEhas closed and after the outlet valve SA has opened (third phase), fluidis ejected from the outlet region A by actuation of the actual pumpmembrane H.

The pump channel can also be fashioned in some other way dependent onthe application. Thus, it is also possible to replace the cross-ribs Qby collar-like walls of round admission and outlet openings A thatproject into the pump channels. In its unexcited condition, the membranesurface then places itself against this collar in a way analogous tothat in which it places itself against the cross-rib and thus closes theadmission or outlet.

Many arrangements are then possible for such a static pump. In accordwith FIG. 13, thus, an ink writing head can be constructed wherein, forexample, nine printing jets S1 through S9 are arranged on a singlesubstrate 1. Each of these printing jets is composed of an admissionvalve SE, of a variable cavity H and of an outlet valve SA. The printingjets S1 through S9 are thereby in communication with the reservoirregion V. In order to be able to fashion a writing head having a greaterplurality of jets, it is also possible to pack a plurality of substrates1 with printing jets on top of one another.

In such an ink writing head, the printing jets S1 through S9 arecompletely functionally separated from the ink supply V. A mechanicalclosure of the jets between writing head and the paper arranged in frontof the writing head can thus be eliminated, as can the drive of thisclosure since the ink channels are closed by the outlet valves SA aslong as these outlet valves SA are not driven. A crosstalk between thejets is eliminated since there is no flow connection during the actualejection event. The ejection events are thereby not limited by thereflection in the ejection channel and are not limited by the crosstalkbetween neighboring jets but are only limited by the intrinsic values ofthe individual printing jets S1 through S9. Air bubbles can beeliminated from the ink channel P by static pumping and empty channelscan be refilled.

As shown in FIGS. 6 through 10, the ink writing head of the inventioncan be manufactured in planar technology in an especially simple way. Tothis end, according to FIG. 6, a substrate 1 composed of piezoceramicand having a thickness of about 200 μm is first polarized and tested.The piezoceramic 1 is then metallized on both sides by vacuum depositionor sputtering (for example, 50 nm Ti and 500 nm Cu). The supportinglayer 2 is then electro-deposited surface-wide. At the same time, theperipheral and central drive electrodes 3 and 4 together with leads Lcan be generated on the opposite side of the ceramic with the assistanceof a photoresist marking (for example, 100 μm Ni).

In order for the button to be able to be separated from the web Q later,a thin intermediate layer Z (for example, 0.2 mm Al or Cu) that isselectively etchable for the remaining structuring is needed in thisregion. It is vapor-deposited or sputtered and is structured withphotolithography etching technique.

The shaping of the channels P can ensue by electro-deposition of a metallayer W between a photoresist structure (for example, 50 μm Ni). Afterapplication of a metallic conductive layer over the non-conductivephotoresist, the carrier layer T is then electro-deposited surface-wide(for example, 100 μm Ni). However, the channel walls W can also beproduced in one step together with the carrier layer T. To that end, astructure K of photoresist or metal (for example, Cu) having the shapeof the later channels and cavities is generated on the supporting layer2. When photoresist is employed, then its surface must be subsequentlyrendered conductive with a further, thin metal layer. Given employmentof metal, the metal of the channel walls [and] carrier layer can bedirectly electro-deposited (for example, 100 μm Ni) on the structure Kand on the remaining, exposed substrate surface 2, FIG. 11.

The structure K of photoresist or metal is then selectively strippedrelative to the overall structure and the cross-web Q, finally, isseparated from the button by dissolving the intermediate layer.

However, it is also possible to first structure the walls of the channelstructure (W) on the thin auxiliary layers (ALU) and to fill an etchablefiller into the channels P formed in this way. After application of thecarrier layer T, the etchable filler is then removed and the auxiliarylayer (Z) is likewise removed, so that the cross-ribs Q can detach fromthe supporting layer 2 when the substrate 1 arcs up.

In order to facilitate the etched removal of the auxiliary layers or,respectively, of the materials that structure the channels, openingsthat are closed later can be provided.

In order to prevent a running [sic] of the composite given temperaturechanges, a further supporting layer SS can be applied on the backside ofthe ceramic layer 1 outside of the electrodes. It is also possible togenerate the lines L for the electrodes 3 and 4 (FIG. 10) simultaneouslywith the supporting layer, i.e. of the same material and in the samethickness.

Instead of the described cross-rib Q, it is also possible according toFIG. 12 to fashion the cross-rib circular, wherewith the axis of theoutlet nozzle A then has a direction perpendicular to the substratesurface 1. It is thus also possible to construct the ink writing headsuch that the outlet nozzles A are arranged at the end face at thesubstrate or such that they are arranged at the substrate surface. Whichis the more advantageous arrangement depends on the application. As isalso shown in FIG. 14, the ink writing head can also be inclined at anangle relative to the scan line in order to increase the divisiondensity between the scan line RZ.

We claim:
 1. Ink writing head comprising ink ejection channels (P) andpiezoelectric transducer elements (3,4) allocated to the ink ejectionchannels (P), said transducer elements being supplied with writing fluidvia supply lines (V), whereby said transducer elements (3,4) contain anelectrically drivable membrane having a first piezoelectricallyexcitable layer (1) and a supporting layer (2) firmly joined to thisexcitable layer and the ink is ejected from the ink ejection channels(P) due to excursions of the membrane,characterized in that thepiezoelectrically excitable layer (1) comprises a peripheral region (3)and a central region (4) that are driven such for generating anexcursion of the membrane needed for writing operation that theperipheral region (3) is shortened by transversal contraction and thecentral region (4) is lengthened by dilation.
 2. Ink writing headaccording to claim 1,characterized in that the piezoelectricallyexcitable layer (1) that is continuously polarized in one directioncomprises a through electrode (2) to ground on its one side andcomprises a first drive electrode (3) allocated to the peripheral regionand a second drive electrode (4) allocated to the central region on itsother side, whereby the peripheral region and the central region arecharged with different electrical fields for driving.
 3. Ink writinghead according to claim 1,characterized in that the piezoelectricallyexcitable layer comprises a through electrode (2) to ground on its oneside and comprises a common drive electrode on its other side, wherebythe peripheral regions and the central region are differently polarized.4. Ink writing head according to claim 1, characterized in that themembrane regions (3,4) of a membrane are arranged concentricallyrelative to one another and arc outward button-like when driven.
 5. Inkwriting head according to claim 1, characterized in that a plurality ofmembrane regions (3,4) individually activatable independently of oneanother are arranged on a common substrate surface (1).
 6. Ink writinghead according to claim 5, characterized in that the drive lines (L) forthe individual membrane regions lead across unpolarized regions of themembrane surface.
 7. Ink writing head according to claim 1,characterized in that the supporting layer (2) is replaced by a furtherpiezoelectrically excitable layer that is respectively polarized in adirection opposite the first piezoelectrically excitable layer.
 8. Inkwriting head according to claim 1, characterized in that the membranemechanically closes the ink channels in its non-activated condition. 9.Ink writing head according to claim 1, characterized in that every inkchannel has three membrane regions that are connected to one another viaa pump channel (P) for the writing fluid allocated to it, said threemembrane regions forming a static pump having two controllable rotarypistons (SE, SA) and a variable cavity, whereby the first membraneregion (SE) is connected, first, to the ink supply system (V) via asupply channel and, second, to the variable cavity (H) and is arrangedas a admission valve between the supply channel and the cavity, thesecond membrane region (PH) effects the pump stroke with its variablecavity, and a third membrane region (SA) is arranged as outlet valvebetween the cavity (PH) and the outlet region (A) of the ink channel.10. Ink writing head according to claim 9, characterized in that thepump channel (P) connecting the membrane regions comprises parting webs(Q) in the region of the membrane surfaces fashioned as valves, saidparting webs cooperating with the membrane surfaces such that, afteroutward arcing of the membrane surfaces, the pump channel opens via theparting webs and, in the non-deflected condition of the membranesurfaces, the pump channel is interrupted in the region of the partingwebs (Q) via the membrane surfaces.
 11. Ink writing head according toclaim 1, characterized in that a plurality of membrane surfaces arearranged over one another in the ink writing head.
 12. Ink writing headaccording to claim 1, characterized in that the ink writing head (TS) isinclined relative to the scan lines in order to diminish the divisionbetween the scan lines (RZ).
 13. Method for the manufacture of an inkwriting head according to claim 5, characterized in that a thin layer ofpiezoceramic is used as substrate (1), the required structure of the inkhead being galvanoplastically constructed thereon.
 14. Method accordingto claim 13, characterized in that the piezoeceramic layer (1) ispolarized before the galvanoplastic structuring.
 15. Method according toclaim 13, characterized in that the piezoceramic (1) is metallized onboth sides by vapor-deposition or sputtering, drive electrodes (3, 4) aswell as the leads (Z) appertaining thereto are subsequentlyphotolithographically-voltaically structured on the one side thereof; inthat, simultaneously, the supporting layer (2) is electro-deposited onthe other side of the piezoceramic; and in that one or more thinauxiliary layers are then generated and structured on the supportinglayer (2), the membrane being capable of being separated from thecross-rib (Q) in the channel structure (P) on the basis of the laterdissolving of said auxiliary layers.
 16. Method according to claim 15,characterized in that the walls (W) of the channel structure (P) areelectro-deposited over the supporting layer (2) and the thin auxiliarylayer (Z) between photoresist structures; in that a thin metal layer isapplied over the photoresist structures and over the channel wall layer;in that the carrier layer (T) is electro-deposited thereon surface-wide;and in that the photoresist is selectively dissolved out.
 17. Methodaccording to claim 16, characterized in that openings that are laterclosed are provided for facilitating the etched removal of the auxiliarylayers or, respectively, of the materials that structure the channels(P).
 18. Method according to claim 15, characterized in that a structure(K) of the photoresist or metal having the shape of the channels isgenerated over the supporting layer (2) and over the thin auxiliarylayers (Z); in that, if the structure (K) is composed of photoresist, athin metal film is applied thereover; in that the channel walls (W) andthe carrier layer (T) are then electro-deposited in common over thesupporting layer (2) and over the structure (K) as a topographic layer;and in that, finally, the structure (K) is selectively dissolved out.19. Method according to claim 15, characterized in that the walls (W) ofthe channel structure for the acceptance of the writing fluid arestructured on the thin auxiliary layers; in that the channels (P) formedin this way are then filled with an etchable filler and a cover layer(T) is applied to the channels (P) filled in this way; and in that thefiller is then removed.